To be honest, my own 'headcannon' involves both major sides. When you feature Fluffy's that care about others and try their best to help, or those who are selfish, it shows depth. The abuse, love (struggle to call it hugbox) ignored, etc, really brings the spectrum of humanity into it too. I've also noticed the 'Smarty' debate, and I think it's also a great thing to include, my headcannon involves them in Feral Fluffies, who, due to harsher environments and radically different diet, have changed fundamentally even if they look the same. It would also make sense that these feral traits (probably brought out most with a survival of the fittest situation for the Ferals) would also pass on to others if Ferals were used in breeding centers as cheap replacements. By that case, only the early generations would not have these traits.
I combine classic fluffy rules with hellgremlin (as you call them) rules to, in my mind, create a layered world of Fluffy life. I also see them as I would any other creature, living, breathing and thinking, though stupid by our definition.
These fictional animals have really made an impact on me, and I have soaked up as much of the wider headcannon as possible to craft my own, a world that by nature is cruel to Fluffy's, but a one in which they can survive regardless of their many flaws. With this, there are so many stories to be told, from the happiest Fluffies, to those tormented to absoluted death. Personalities will always shine through, which each creature being it's own being, regardless of the original intention's in their creation.
Only stuff I don't include is weirdbox stuff or something that makes them appear more toylike over animal like (though I do include their speech as something hard coded into their genetics, but is still affected by their environment, even before birth.)